Method of producing underwater seismic waves and apparatus therefor

ABSTRACT

A method of producing seismic waves under water is provided. A seismic wave generator is immersed beneath the surface of the water. A continuously varying predetermined command signal is generated for a period of time to operate a motor. The motor, operated in accordance with the command signal, controls the flow of pressurized fluid from the seismic wave generator into the surrounding water. A continuously varying pressure wave is thereby generated in the surrounding water. Changes in pressure in the surrounding water are detected and a feedback signal is generated in accordance with the changes in pressure. The feedback signal is combined with the command signal to produce a control signal which operates the motor to generate the desired pressure wave in the surrounding water.

United States Patent 1191 Farron et al. 5] Jan. 16, 1973 [54] METHOD OFPRODUCING 3,481,426 12/1969 Silverman ..l8l'/0.5 H UNDERWATER SEISMICWAVES AND 3,322,232 5/1967 Chalmers et a! ..l8l/0.5 H APPARATUS THEREFORP E Y B A B h 1 rzmary xammer en amm orc e t [75] Inventors: .iolan R.Ilarrgni Mishiizwakia, 31d: Assistant Binnie! n few e oya a Attorne ReedC. Lawlor Mich.; Matthew Slavin, Pasadena, y Calif.; Bernard R.Teitelbanm, Bir- 57 ABSTRACT mingham, Mich. A method of producingseismic waves under water is Asslgneei Unlted p y Corporatlon, provided.A seismic wave generator is immersed Pasadena, C3115. beneath thesurface of the water. A continuously vary- [22] Filed: Jam 15, 1971 ingpredeterr nined command signal is generated for a penod of time tooperate a motor. The motor, pp ,772 operated in accordance with thecommand signal, controls the flow of pressurized fluid from the seismic[52] Cl 340/7 R, 181/05 H, 181/05 VM, wave generator into thesurrounding water, A continu- 3 40/17 ously varying pressure wave isthereby generated in [511 1111. C1. ..H04b 13/00, GOlv 1/02 thesufmmding water- Changes f F f 581 Field of Search ..1s1/0.5 H, 0.5 VM;340/17, mmdmg l are sgnal 340/12 7 generated in accordance with thechanges in pressure. The feedback signal is combined with the command[56] References Cited signal to produce a control signal which operatesthe motor to generate the desired pressure wave in the UNITED STATESPATENTS Surrounding Water- 3,369,519 2/1968 Bricout l8l/0.5 H 24 Claims,8 Drawing Figures METHOD OF PRODUCING UNDERWATER SEISMIC WAVES ANDAPPARATUS THEREFOR BACKGROUND OF THE INVENTION This invention relates toa method of generating seismic waves under water, and more specificallyto a method of generating sesimic waves under water for underwaterseismic exploration, and to an apparatus for use in carrying out themethod.

The most popular methods currently employed for generating underwaterseismic waves utilize explosions, the release of highly pressurized airfrom a pneumatic source, an underwater electrical discharge whichcreates an expanding bubble of ionized gas, or the sudden movement of aflexible or rigid member against the water. Typically, each of thedevices employed in these methods is capable of creating only one formof pressure wave in the water. Also, a substantial time delay isincurred before such a device can again be used to create anothersimilar pressure wave.

For example, when dynamite is used as the source of energy, a separatecharge is required for each pressure wave to be generated. Thegeneration of multiple pressure waves requires -a time delay forrecharging the device used or requires the use of an array of multiplecharges which can be exploded in succession. Further, the pressure wavegenerated will depend in part on the size of the explosive charge. Somedevices can operate with only a charge of limited size, thereby beingcapable of producing only a limited range of pressure waves in aparticular environment.

Pneumatic sources currently being employed use sealed containers whichare pressurized with air and then suddenly opened under water to ventthe pressurized air into the water. This creates an expanding bubble ofair in the water and a resulting pressure wave which is dependent uponthe pressure of the air originally within the sealed container. Anotherpressure wave cannot be generated until the chamber is again sealed andpressurized. This recharging requires a substantial time delay. Also,such devices frequently discharge only when one pressure is reached,thus being capable of producing only one form of pressure wave in aparticular environment.

This invention overcomes some of the difficulties encountered in the useof the prior art seismic wave sources by providing an underwater seismicwave source which is capable of continuously flowing varying amounts ofpressurized fluid, such as air or water, into the surrounding water,thereby producing an output consisting of continuously varying pressurewaves in the surrounding water.

As used herein, a wave means a series of at least three connectedalternations. Flow or rate of flow means the volumetric rate of flow ofafluid.

A further advantage of this invention is that the pressure wave outputis controlled by a feedback system which enables the operator to obtainthe desired output.

Another advantage of this invention is that it is an open system device.That is, the pressurized fluid is expelled from the seismic wavegenerator into the surrounding water rather than being recycled andstored. This allows fluids, such as air or sea water, to be utilized andeliminates the necessity for employing expensive hydraulic fluid and thelike which require cooling and storage equipment. Furthermore, air andsea water are in relatively endless supply to such a seismic wavegenerator and may be pumped directly from the environment surroundingthe seismic wave generator.

The best embodiment of the invention provides an output which, if a gasis employed as the pressurized fluid, will produce small bubbles in thesurrounding water, thus reducing the occurrence of the secondarypressure pulses associated with the collapse of large bubbles. This isaccomplished by providing a plurality of outlet tubes, each having asmall diameter so that the bubbles ejected from these outlet tubes willhave a small volume.

SUMMARY OF THE INVENTION This invention provides a method for generatingseismic waves under water which comprises the steps of generating acontinuous command signal that varies as a function of time in apredetermined or desired manner, flowing a fluid into the water andvarying the flow of the fluid into the water in accordance withvariations in the command signal, thereby generating an alternatingpressure wave in the water, generating a feedback signal in accordancewith the flow of fluid, and controlling the flow of fluid in accordancewith a combination of the command and feedback signals.

An apparatus for performing the steps of this method comprises a commandsignal generator, a motor adapted to be actuated by command signals fromthe command signal generator, a housing having an inlet and outletorifice, control means adapted to be actuated by the motor to vary thefl-ow of fluid from the housing, and feedback means adapted to detectchanges in the flow of fluid out of the housing and to generate afeedback signal in accordance therewith. The feedback means is connectedin order to combine the feedback and command signals to produce acontrol signal which controls the flow of fluid out of the housing.

Many other objects and advantages will become evident to those skilledin the art upon a reading of the following detailed description and!drawings wherein:

FIG. 1 is a schematic drawing of a system for employing the invention;

FIG. 2 is a schematic drawing of one embodiment of the invention;

FIG. 3 is a drawing representing a magnetic tape adapted to be used inone embodiment of this invention;

FIG. 4 is a time plot of a typical signal generated by the magnetic tapeshown in FIG. 3;

FIG. 5 is a cross-sectional plan view of an outlet of one embodiment ofthis invention;

FIG. 6 is a sectional elevational view of a part of another type ofseismic wave generator which may be employed in this invention;

FIG. 7 is a sectional elevational view of a part of another type ofseismic wave generator which may be employed in this invention; and

FIG. 8 shows an alternative position for the pressure transduceremployed in one embodiment of this invention.

FIG. 1 shows a system for employing the methods and apparatus of thisinvention. This system includes a powered boat BT, seismic wave pickups,such as hydrophones or geophones PU, and a submerged seismic wavegenerator SG. The pickups PU are towed by the boat BT with electricalcable EC 1. The boat BT tows the seismic wave generator 86 with a towline TL. Seismic wave generator 80 is connected with the controlequipment CE by electrical cable EC2. The boat BT carries controlequipment CE for generating command signals to actuate the seismic wavegenerator SG and for receiving and recording signals from the string ofseismic wave pickups PU. The boat BT also carries a fluid pump FP, suchas an air compressor, which is connected to the seismic wave generatorSG by a fluid conduit FC in order to supply pressurized fluid to theseismic wave generator SG.

As the boat BT tows the seismic wave generator SG and the pickups PUthrough the water, pressurized fluid is supplied to the seismic wavegenerator SG and command signals are generated by the control equipmentCE. The command signals actuate the seismic wave generator SG to releasecontrolled amounts of pressurized fluid into the surrounding water in apredetermined manner, thereby generating pressure waves in the water.These pressure waves produce seismic waves in the earth strata beneaththe water. The seismic waves are deflected back through the watertowards the surface of the water. Seismic waves resulting from thesepressure waves are detected by the seismic wave pickups PU whichgenerate signals in accordance therewith. These signals are returnedthrough seismic cable SC to the control equipment CE where they arerecorded.

FIG. 2 shows the best embodiment of the seismic wave source of thisinvention presently known and various mechanical and electricalcomponents of this embodiment.

CONTROL EQUIPMENT In the embodiment shown in FIG. 2, a constant driveelectric motor 100 is employed to drive a drum 102 which is keyed todrum shaft 104. A magnetic tape 110 is secured to the rotating drum 102.Portions of the magnetic tape 110 are magnetized in order to generate apredetermined command signal when passed near magnetic reproducing orreading head 120. The magnetic reading head 120 consists of aninductance coil which generates an alternating current in accordancewith the variations in magnetization of the magnetic tape 1 when themagnetic tape 1 10 is passed near the reading head 120, as is well knownto those skilled in the art.

In FIG. 3. there is shown a magnetic tape 110 which has recorded thereona recording of a wave which is to be employed to produce a seismic waveof desired characteristics. In the specific embodiment of the inventionillustrated, the recording is in the form of a frequency-modulated wavein which the frequency of the wave at any point deviates from areference or carrier frequency by an amount proportional to theamplitude of the desired seismic wave. The wave itself is indicated onlyschematically on the magnetic tape 110. A series of such recordings arerecorded on the tape 110. As indicated in FIG. 3, each recording extendsover a time interval of 3.5 seconds and successive recordings areseparated by silent intervals of 3.0 seconds. The actual intervals aredetermined by the geometry of the system and the speed of operation ofthe motor 100. The time intervals specified correspond to typicalsituations. The carrier frequencyis very high, such as 10,000 hz.,compared with the dominant frequencies of the seismic waves to beproduced.

As the drum is rotated in a clockwise direction with the tape mounted onthe drum 102 as illustrated in FIG. 2, a frequency-modulated electricalwave is generated in the output of the reproducing head 120. This signalis demodulated by means of a discriminator network 124 in order toproduce at its output an electrical copy 124A of the originalfrequency-modulated signal. In the particular example, this outputsignal 124A increases in frequency as a function of time as indicatedwith reference to the increasing time vector 125.

Such a command signal may also be produced in other ways and may varywith time in other ways. For example, if a signal of constant amplitudebut increasing frequency is recorded on tape 110, a command signalhaving an increasing amplitude and frequency may be produced at theoutput of reading head requiring the use of a low pass filter in placeof discriminator 124 in order to obtain a command signal having aconstant amplitude and an increasing frequency.

FIG. 4 shows a typical series of command signals 124A, which may be inthe form of electrical signals or other physical phenomenon, generatedby rotation of the drum 102 near reproducing head 120. A signal 124A ofincreasing frequency is generated for approximately 3.5 seconds whilethe magnetic tape 110 passes across the magnetic reading head 120. Thenno signal is generated at the output of the discriminator 124 forapproximately the next 3.0 seconds while a corresponding part of themagnetic tape 110 passes across the magnetic reading head 120.

Thus, one such signal 124A of approximately 3.5 seconds duration isgenerated during each 6.5 second interval. As will be described below,this signal will result in a pressure wave having a constant amplitudeand an increasing frequency. Of course, the tape 110 may be magnetizedin many other ways to produce many other command signals resulting invarious types of pressure waves. Such pressure waves may take variousforms, such as a wave having a constant amplitude or an alternating wavehaving a constant frequency.

The electrical command signal 124A which remains after passing throughdiscriminator 124 has a constant amplitude and a frequency whichincreases as a linear function of time. This is referred to hereinafteras a chirp type command signal and the frequency range which will beused herein as an example will be from 20 to 60 hertz. Electricalcommand signal 124A is a representation of the pressure wave outputwhich is derived from the seismic wave generator SG.

In this system the command signal operates to produce a pressure wavejust outside the plenum chamber PC which corresponds to the commandsignal. This is accomplished in part by the action of a servo motor 140which has a rotor R0 and a stator ST. The command signal 124A is passedthrough the power amplifier which applies the command signal inamplified form to the stator.

An electrical signal 152A corresponding to the pressure wave is producedby the transducer 154 and after passage through a compensating network170 and an amplifier 180, the modified signal 180A is applied, alongwith the command signal 124, to the opposite input sections of adifferential amplifier 126. The resultant amplified difference signal126A is further amplified by a power amplifier 128 to produce anamplified difference signal 1268 which is supplied to the rotor R0. Withthis arrangement the valve is moved inwardly and outwardly as the casemay be to cause the amplified signal 180A to match the command signal124 and hence to control the pressure outside the plenum chamber in apredetermined manner.

Seismic Wave Generator Seismic wave generator 86 consists of a surgechamber SU having an inlet adapted to receive fluid from fluid conduitFC, control valve housing CH, and plenum chamber PC having outlets 148communicating with the surrounding water. Pressurized fluid from thefluid pump FP is conducted into the surge chamber SU by fluid conduitPC. In the best embodiment of this invention, servo motor 140 in thecontrol valve housing CH is actuated by control signals 126B and commandsignals 124A. Servo motor 140 controls the position of valve 144 whichin turn controls the passage of pressurized fluid from the control valvehousing CH into the surrounding water.

The surge chamber SC receives highly pressurized fluid from the fluidpump PP and acts as a safety to guard against imposing unduly highpressures in the rest of the fluid supply system when pressurized fluidbacks up in the system due to a sudden restriction between the surgechamber SU and the plenum chamber PC caused by the valve 144.

In this embodiment of this invention, the control valve housing CHhouses a servo motor 140 and a valve 144. Valve 144 is connected toservo motor 140 through gearing which acts to move the valve 144 towardsand away from the valve seat 146 provided by control valve housing CH.The valve seat 146 is actually a concave annular shoulder of controlvalve housing CH. Valve 144, actuated by servo motor 140, thus regulatesthe flow of pressurized fluid from the surge chamber SU to the plenumchamber PC.

As the rate of flow of the fluid varies, the pressure of fluid in theplenum chamber PC and the surrounding water also varies accordingly.

In the best embodiment of this invention, the valve 144 never entirelystops the flow of fluid into the plenum chamber PC while the seismicwave generator 50 is under water. This keeps the surrounding water fromflowing into the seismic wave generator SG. Thus, the seismic wavegenerator SC is always operated under water so that the pressure in theplenum chamber PC is maintained slightly above the pressure of thehydrostatic head of the surrounding water. Increased pressurized fluidis supplied to the plenum chamber PC in order to generate the desiredpressure waves in accordance with the control signal 1268 and commandsignal 124A.

The plenum chamber PC, shown in FIGS. 2 and 5, is a hollow sphericalshell connected at one end to the control valve housing CH in order toreceive pressurized fluid therefrom. In the best embodiment of thisinvention, a series of small diameter outlet tubes 148 are symmetricallypositioned around the equator of the plenum chamber PC as outlets forthe seismic wave generator SG. Each of these outlet tubes 148 isapproximately 6 inches in length and! about l xinches in diameter. Sucha construction provides an outlet for the pressurized fluid which, ifthe pressurized fluid employed is gas, will reduce the size of thebubbles emitted and diffuse the gas bubbles. This reduces the occurrenceof secondary pressure pulses associated with the collapse of largebubbles. Also, the reaction forces generated by the flow of thepressurized fluid from the plenum chamber PC balance each other due tothe symmetrical positioning of the outlet tubes 148 so that theunderwater parts of the seismic wave generator 86 will not be movedlaterally by the force of the pressurized fluid being expelled into thesurround ing water. In this embodiment of the invention, the outlets 148are maintained close enough together so that they may all be consideredas a single point source from a distance of about 8 feet.

Feedback A pressure transducer 152, shown in FIG. 2, is secured to theseismic wave generator 86 by arm 153. In the best embodiment of theinvention, the pressure transducer 152 is located outside of the plenumchamber PC in order to detect changes in pressure in the water adjacentto the plenum chamber PC. Such pressure changes will occur over a periodof time as varying amounts of pressurized fluid are emitted from theoutlet tubes 148. In this embodiment of the invention, the pressure atthe transducer 152 will often be at or above the hydrostatic pressure atthe depth of the transducer since the pressure transducer will beremoved from any collapsing bubbles and will be at approximately thesame depth as the outlets 148.

Pressure transducer 152 is located less than approximately one-eighth ofa minimum dominant wave length away from the orifice of the nearestoutlet tube 148. Therefore, where a 20 to 60 hertz chirp command signalis employed, the pressure transducer should be located approximately 8feet away from the orifice of the nearest outlet tube 148.

In the best embodiment of this invention, the pressure transducer 152 isa piezoelectric crystal secured to a housing 154, as shown in FIG. 2. Aportion of piezoelectric crystal 152 protrudes from the housing 154 inorder that the piezoelectric crystal 152 will be affected by thepressure waves in the surrounding water. Two electrodes (not shown) aresecured to two opposite faces of the piezoelectric crystal 152 inside ofthe housing 154.

The piezoelectric crystal 152 generates an alternating feedback signal152A when the crystal 152 is subjected to variations in pressure. Thesignal 152A is passed through the compensating network generallydesignated as 170, compensates for changes in phase between the signals1268 and 152A and generates signal A which is approximately out of phasewith command signal 124A over the frequency range employed. Amplifier180 receives signal 170A and generates amplified feedback signal 180A.Signal 180A is a representation of the pressure wave output of theseismic wave generator SG detected by pressure transducer 152. Signals124A and 180A are combined by amplifier 126, as is well known in theart, to produce control signal 126A. I

Control signal 126A, which is the difference between command signal 124Aand feedback signal 180A, drives servo motor 140 to correct fordistortions in the system. Thus, control signal 126A corrects fordeviations of the detected output pressure wave from the desired outputpressure wave.

Operation A 180 phase shift occurs in an underwater pressure wave whenit is reflected from the surface of the water. This phase shift operatesto produce a zero net output for the seismic wave generator if theoutlet 148 is positioned one-half wave length below the surface of thewater. The outlet of the seismic wave generator SG of this inventionshould therefore be positioned at approximately one-fourth of a minimumwavelength below the surface of the water when in operation.

The wavelength of a pressure wave in water is equal to the velocity ofthe wave in the water divided by the frequency of the wave. The velocityof a pressure wave in water is approximately 5,000 feet per second. Themaximum frequency in the example previously considered was 60 hertz.Therefore, one-fourth of the wavelength is equal to 5,000 divided by (4X 60). This equals approximately 21 feet which is the depth below thesurface of the water at which the outlets 148 of the seismic wavegenerator 86 should be positioned when the seismic wave generator SG isin operation.

First Alternative Embodiment Another embodiment of the invention employsthe apparatus shown and described in US. Pat. No. 3,105,671 issued toBernard R. Teitelbaum and Albert Blatter on Oct. 1, 1963. A modifiedversion of this apparatus is shown in FIG. 6. This apparatus may beemployed in place of the seismic wave generator SG described above. US.Pat. No. 3,105,67l (herein I referred to as the Teitelbaum patent) ishereby incorporated herein by reference in its entirety.

In the device shown in FIG. 6, the electrical signal received by input22 of torque motor is command signal 124A. The pressure source 44referred to in the Teitelbaum patent is the high pressure fluid suppliedby fluid pump PP and fluid conduit FC referred to above, and shown inFIG. 6. The housing 40 of the Teitelbaum patent is control valve housingCH. Housing 40 of Teitelbaum is extended to enclose torque motor 20, asshown in FIG. 6. The extension of housing 40 provides protection formotor 20 and for its linkage to valve or flapper 36 and to diaphragm 74.Orifice 48 is exposed to the pressure in the surrounding fluid medium byorifice 80 provided in the extension of housing 40.

The chamber 66 in this embodiment corresponds to the plenum chamber PCand the nozzle 70 corresponds to outlet tubes 148 of the previouslydescribed embodiments. The feedback system in this alternativeembodiment consists of passage 72 which connects chamber 66 to diaphragm74. Fluid pressure in chamber 66 thereby acts on diaphragm 74 which inturn moves arm 30, through link 76, to resist or to reinforce the forcetransmitted from motor 20 to valve 36.

As is more fully described in the Teitelbaum patent, in the operation ofthis embodiment, command signal 124A is received at input 22 by motor 20and a corresponding torque is applied to arm 24. This rotates arm 30and, through a mechanical link, valve 36 against the bias of spring 34.Thus, the torque applied 'to arm 30 by motor 20 becomes a command signalfor valve 36. The rotation of arm 30 also moves link 76. The movement ofvalve 36 away from orifice 42 and towards orifice 48 allows morepressurized fluid to flow into chamber 39, allowing a greater pressureto be developed in chamber 39. The pressure in chamber 39 is transmittedthrough passage 49 to diaphragm 50. This tends to move valve 52 againstthe bias of spring 58 and circumferential rim 54 of valve 52 out ofcontact with valve -seat 56. When the pressure on diaphragm 50 is greatenough, the seal between rim 54 and valve seat 56 will open, allowingpressurized fluid from the fluid conduit FC to enter the chamber 66 andpass through nozzle 70.

The pressure in chamber 66 is transmitted through passage 72 todiaphragm 74. The force on diaphragm 74 biases arm 30 through link 76.The action of the torque motor 20 on arm 30 must overcome this bias inorder to move valve 36. In this manner, a feedback signal is providedwhich is combined with the command signal, that is, the torque appliedby motor 22 to arm 30, to provide a resultant control signal whichactuates valve 36. Thus, the pressure transducer 152, compensatingnetwork 170, and amplifier 180 are not necessary in this embodiment ofthe invention.

Second Alternative Embodiment In a second alternative embodiment of thisinvention, shown in FIG. 7, a torque motor 20, as described inTeitelbaum, is employed in place of the servo motor 140 previouslydescribed. The torque motor 20, in this embodiment, receives controlsignal 1268, as in the best embodiment of this invention. Signal 1268operates valve 36 as described above under the first alternativeembodiment, except that passage 72, diaphragm 74, and link 76 have beenomitted.

In this embodiment of the invention, control signal 126B actuates valve36 without interference from link 76. Feedback is provided by means ofpressure transducer 152 located outside of housing 40, as is shown inFIG. 7 and as described above in connection with the best embodiment. a

In this alternative embodiment of the invention, when no command signal124A is being generated, a holding signal (not shown) is supplied to theseismic wave generator SG to maintain a small flow of fluid from theseismic wave generator SG. This keeps the surrounding water from flowinginto the seismic wave generator SG.

Alternative Position of Pressure Transducer In this alternativeembodiment, pressure transducer 152 is mounted within the plenum chamberPC in order to detect changes in the pressure within plenum chamber PC.Feedback signal 152A is generated in accordance with the change inpressure in the plenum chamber PC and will correct for deviations of thepressure in the plenum chamber PC from the desired pressure, aspreviously described.

Similarly, pressure transducer 152 may be mounted in chamber 66 ornozzle of the second alternative embodiment (not shown).

All of the embodiments of the invention herein disclosed have thecapability of producing pressure waves in water which have amplitudes orfrequencies or both which vary as a function of time. Furthermore, thefeedback feature of this invention provides a control feature lacking inthe underwater seismic wave sources presently in use. This feedbackfeature has the advantage of providing pressure waves in the surroundingwater which may be made to be almost identical to each other, thusexpediting the correlation of these waves. The fact that this inventiondoes not require that pressurized fluid be cooled and stored allowseconomical operation of the underwater seismic wave source. For example,sea water may be pumped by fluid pump FP directly from the ocean intothe control valve housing CH by way of fluid conduit FC.

Although this invention has been described with reference to particularapplications, the principles involved are susceptible to numerous otherapplications which will be apparent to persons skilled in the art andthe scope of the invention is not to be limited to the precedingembodiments.

The invention claimed is:

l. A method of generating seismic waves under water for underwaterseismic exploration which comprises:

generating a continuous predetermined command signal that varies as afunction of time;

causing a fluid to flow into the water in accordance with variations ofsaid command signal, thereby generating a varying pressure wave in thewater;

generatinga feedback signal in accordance with said flow of fluid; and

controlling said flow of fluid in accordance with a combination of saidcommand and feedback signals.

2. The method of claim 1 wherein said command signal is generated with afrequency which varies as a function of time.

3. A method of generating seismic waves under water for underwaterseismic exploration comprising the steps of:

immersing a seismic wave generator in a fluid medigenerating apredetermined command signal in order to produce a desired pressure waveoutput; generating a pressure wave output in the fluid medium byactuating said seismic wave generator in response to said commandsignal;

detecting said pressure wave output;

generating a feedback signal in accordance with said detected pressurewave output; and

modifying the actuation of said seismic wave generator in accordancewith said feedback signal to produce said desired pressure wave output.

4. The method of claim 3 wherein said generated command signal varies asa function of time and the pressure wave output varies in accordancetherewith.

5. The method of claim 3 wherein said pressure wave output is modifiedby combining said command signal and said feedback signal to produce acontrol signal which is sued to generate the desired pressure waveoutput.

6. The method of claim 5 further comprising the steps of:

pumping water from the surrounding water into said seismic wavegenerator under pressure; and generating a pressure wave in thesurrounding water by releasing controlled amounts of said pressurizedwater from the seismic wave generator into the surrounding water at arate that varies with the magnitude of said control signal.

7. A method generating seismic waves under water for underwater seismicexploration comprising of steps of:

Step 1. immersing a seismic wave generator, having an inlet orifice andan outlet orifice, into a fluid medium;

Step 2. generating a continuous predetermined command signal that variesas a. function of time in order to produce a desired pressure wave;

Step 3. continuously supplying fluid under high pressure to the inlet ofsaid seismic wave generator;

Step 4. continuously venting said pressurized fluid from the outlet ofsaid seismic wave generator into the fluid medium surrounding theseismic wave generator in response to said command signal, therebygenerating a continuously varying pressure wave in said surroundingfluid medium;

Step 5. continuously detecting changes in the flow of fluid from saidseismic wave generator;

Step 6. generating a feedback signal in accordance with said detectedpressure;

Step 7. combining said feedback signal with said command signal in orderto produce a control signal;

Step 8. generating the desired pressure wave in accordance with saidcontrol signal;

Step 9. maintaining the flow of pressurized fluid from said outlet atall times;

Step 10. said pressure wave creating seismic waves that travel toportions of the earth from which they are deflected to the surface;

Step 1 1. receiving and recording such deflected waves; and

Step 12. repeating steps 2 through 1 l.

8. The method of claim 7 wherein step 5 comprises continuously detectingchanges in pressure in the fluid medium surrounding the seismic wavegenerator at a point spaced from said outlet orifice through which thepressurized fluid is vented.

9. The method of claim 7 wherein step 9 comprises reducing the amplitudeof said command signal to maintain a small flow of pressurized fluidfrom said outlet.

10. In a method of generating seismic waves under water for underwaterseismic exploration, wherein pressure waves are developed in the waterwhich act on earth strata beneath the water to generate seismic waveswhich are in turn deflected towards the surface of the water anddetected, the steps of:

immersing a seismic wave generator into the surrounding water;

moving said seismic wave generator and a seismic wave detector throughthe water while immersed;

generating a predetermined command signal that varies as function oftime;

operating a motor within said seismic wave generator in response to acontrol signal;

generating a pressure wave in the water in response to the operation ofsaid motor which pressure wave varies in accordance with variations insaid control signal;

detecting said pressure wave in the water adjacent to said seismic wavegenerator;

generating a feedback Signal in accordance with said detected pressurewave;

continuously combining said command signal and said feedback signal inan amplifier to produce said control signal which will generate thedesired pressure wave;

detecting seismic waves produced by said pressure wave and deflectedfrom the earth strata beneath the surrounding water with said seismicwave detector; and

recording indications of said detected seismic waves.

11. The method of claim wherein said seismic wave generator is submergedto a depth of about 21 feet below the surface of the water.

12. An underwater seismic wave source comprising:

a command signal generator adapted to generate a command signal thatvaries as a function of time; motor means driven by said command signal;

a housing having orifice means for passing pressurized fluid into andout of said housing;

control means adapted to be actuated by said motor means to vary theflow of fluid out of said housing; and

feedback means adapted to detect changes in the flow of fluid out ofsaid housing and to generate a feedback signal in accordance with saidflow, said feedback means being connected to combine said feedbacksignal with said command signal to produce a control signal whereby saidflow of fluid out of said housing varies in accordance with said controlsignal.

13. An underwater seismic wave source as defined in claim 10 whereinsaid control means comprises a valve actuated by said motor means andadapted to restrict the flow of fluid out of said housing.

14. An underwater seismic wave source as defined in claim 13 whereinsaid feedback means further comprises combining means and a transducer,said transducer being positioned outside of said housing at a distanceless than about 8 feet from said orifice and being adapted to respond tochanges in said flow of fluid out of said housing, said transducer beingconnected to supply said feedback signal to said combining means inorder to combine said command signal and said feedback signal to producea control signal to drive said motor means.

15. An underwater seismic wave source as defined in claim 13 whereinsaid feedback means is connected to supply said feedback signal tomodify the response of said control means to said motor means.

16. In an underwater seismic wave source having a command signalgenerator, motor means adapted to be driven by command signals, ahousing having an inlet for passing pressurized fluid into said housingand a plurality of outlets for passing pressurized fluid out of saidhousing, and a valve adapted to drive by said motor means to restrictthe flow of fluid out of said housing, the combination therewith of:

a pressure transducer adapted to detect changes in pressure adjacent tosaid housing outlets and to generate a feedback signal in accordancewith said pressure changes, said pressure transducer being connected tosupply said feedback signal to control said flow of fluid out of saidhousing.

17. An underwater seismic wave source comprising:

a command signal generator adapted to continuously generate commandsignals that vary as a function of time;

a housing having a fluid inlet and a plurality of tubular fluid outlets,said outlets being positioned in close proximity to each other;

valve means positioned in said housing adapted to control the rate offlow from said inlet to said outlets;

motor means connected to said command signal generator adapted to bedriven by said command signals to move said valve means into and out ofrestrictive engagement with said housing in order to vary the flow offluid through said housing; and

a pressure transducer adapted to detect changes in the flow of fluid outof said housing and to generate feedback signals in accordancetherewith, said pressure transducer being connected to combine saidfeedback signals with said command signals to produce control signals,said flow of fluid out of said housing varying in accordance with saidcontrol signals.

18. An underwater seismic wave source as defined in claim 17 whereinsaid motor comprises a servo motor and wherein said pressure transduceris mounted outside of said housing adjacent to said fluid outlets;

said pressure transducer being connected to combine said feedbacksignals and said command signals to produce control signals foractuating said servo motor.

19. An underwater seismic wave source as defined in claim 17 whereinsaid pressure transducer is mounted inside of said housing adjacent tosaid fluid outlets in order to detect the pressure inside of saidhousing adjacent to said fluid outlets.

20. An underwater seismic wave source as defined in claim 17 whereinsaid motor comprises a torque motor and said feedback means comprises apressure transducer mounted inside of said housing adjacent to saidfluid outlets;

said pressure transducer being connected to combine said'feedbacksignals and said command signals to produce control signals foractuating said torque motor.

21. An underwater seismic wave source as defined in claim 17 furthercomprising combining means and wherein said pressure transducer isadapted to detect changes in pressure outside of said housing, adjacentto said outlets and to generate feedback signals in accordance with saidpressure, said pressure transducer being connected to supply feedbacksignals to said combining means in order that said command signals andsaid feedback signals may be combined to produce control signals tofurther drive said motor means.

22. An underwater seismic wave source comprising:

a command signal generator adapted to continuously generate commandsignals that vary in frequency as a function of time;

a housing having a fluid inlet and a fluid outlet;

valve means positioned in said housing adapted to provide a seal betweensaid inlet and said outlet, said housing providing a valve seat forengagement by said valve;

said valve means having an intermediate radially extending flangeadapted to engage said valve seat,

said valve means being connected to said housing by upper and lowerdiaphragm sealing means;

means resiliently urging said valve means into restrictive engagementwith said valve seat;

said housing having a chamber therein having two closely spacedorifices, one orifice communicating with the outside of the housing andthe other communicating with said fluid inlet;

flapper means disposed between said orifices and being adapted torestrict the flow of fluid through one of said orifices at a time;

motor means connected to said command signal generator and adapted tomove said flapper means in accordance with command signals received fromsaid generator, thereby controlling pressure in said chamber, saidchamber communicating with the outer side of one of said diaphragms tocontrol the position of said valve means connected to said one diaphragmby allowing pressure to be exerted upon said one diaphragm to move saidvalve against said resilient means and out of engagement with said valveseat; and

feedback means adapted to detect changes in pressure adjacent to saidhousing outlet and to generate feedback signals in accordance therewith,said pressure transducer being connected to modify the movement of saidflapper means.

23. An underwater seismic wave source comprising:

a rotating drum;

a magnetic tape secured to said drum, said tape having zones magnetizedwith a polarity opposite from the polarity of other zones of the tape;

a magnetic tape reading head positioned adjacent to said rotating drum,said reading head being adapted to generate electric command signals inresponse to the passage of zones of varying magnetization near saidreading head;

a housing having a fluid inlet and a plurality of tubular fluid outlets;

a motor mounted in said housing and connected to receive and be drivenby said electric command signals;

a valve positioned in said housing adapted to provide a fluidrestriction between said fluid inlet and said fluid outlets, saidhousing providing a valve seat for engagement by said valve;

a surge chamber adapted to receive highly pressurized water pumped fromthe surrounding water through one end of said surge chamber;

said motor being adapted to move said valve into and out of engagementwith said valve seat to release controlled amounts of said pressurizedwater into the surrounding water;

a plenum chamber positioned between said valve and said outlets; and

feedback means secured to said housing and being adapted to detectchanges in. pressure adjacent to said housing outlets and to generatefeedback signals in accordance therewith, said feedback means beingconnected to modify the flow of pressurized water from said housing inaccordance with said feedback signals.

24. The method of claim 7 wherein said changes in the flow of fluid aredetected by detecting changes in the pressure of the fl uid

1. A method of generating seismic waves under water for underwaterseismic exploration which comprises: generating a continuouspredetermined command signal that varies as a function of time; causinga fluid to flow into the water in accordance with variations of saidcommand signal, thereby generating a varying pressure wave in the water;generating a feedback signal in accordance with said flow of fluid; andcontrolling said flow of fluid in accordance with a combination of saidcommand and feedback signals.
 2. The method of claim 1 wherein saidcommand signal is generated with a frequency which varies as a functionof time.
 3. A method of generating seismic waves under water forunderwater seismic exploration comprising the steps of: immersing aseismic wave generator in a fluid medium; generating a predeterminedcommand signal in order to produce a desired pressure wave output;generating a pressure wave output in the fluid medium by actuating saidseismic wave generator in response to said command signal; detectingsaid pressure wave output; generating a feedback signal in accordancewith said detected pressure wave output; and modifying the actuation ofsaid seismic wave generator in accordance with said feedback signal toproduce said desired pressure wave output.
 4. The method of claim 3wherein said generated command signal varies as a function of time andthe pressure wave output varies in accordance therewith.
 5. The methodof claim 3 wherein said pressure wave output is modified by combiningsaid command signal and said feedback signal to produce a control signalwhich is sued to generate the desired pressure wave output.
 6. Themethod of claim 5 further comprising the steps of: pumping water fromthe surrounding water into said seismic wave generator under pressure;and generating a pressure wave in the surrounding water by releasingcontrolled amounts of said pressurized water from the seismic wavegenerator into the surrounding water at a rate that varies with themagnitude of said control signal.
 7. A method generating seismic wavesunder water for underwater seismic exploration comprising of steps of:Step
 1. immersing a seismic wave generator, having an inlet orifice andan outlet orifice, into a fluid medium; Step
 2. generating a continuouspredetermined command signal that varies as a function of time in orderto produce a desired pressure wave; Step
 3. continuously supplying fluidunder high pressure to the inlet of said seismic wave generator; Step 4.continuously venting said pressurized fluid from the outlet of saidseismic wave generator into the fluid medium surrounding the seismicwave generator in response to said command signal, thereby generating acontinuously varying pressure wave in said surrounding fluid medium;Step
 5. continuously detecting changes in the flow of fluid from saidseismic wave generator; Step
 6. generating a feedback signal inaccordance with said detected pressure; Step
 7. combining said feedbacksignal with said command signal in order to produce a control signal;Step
 8. generating the desired pressure wavE in accordance with saidcontrol signal; Step
 9. maintaining the flow of pressurized fluid fromsaid outlet at all times; Step
 10. said pressure wave creating seismicwaves that travel to portions of the earth from which they are deflectedto the surface; Step
 11. receiving and recording such deflected waves;and Step
 12. repeating steps 2 through
 11. 8. The method of claim 7wherein step 5 comprises continuously detecting changes in pressure inthe fluid medium surrounding the seismic wave generator at a pointspaced from said outlet orifice through which the pressurized fluid isvented.
 9. The method of claim 7 wherein step 9 comprises reducing theamplitude of said command signal to maintain a small flow of pressurizedfluid from said outlet.
 10. In a method of generating seismic wavesunder water for underwater seismic exploration, wherein pressure wavesare developed in the water which act on earth strata beneath the waterto generate seismic waves which are in turn deflected towards thesurface of the water and detected, the steps of: immersing a seismicwave generator into the surrounding water; moving said seismic wavegenerator and a seismic wave detector through the water while immersed;generating a predetermined command signal that varies as function oftime; operating a motor within said seismic wave generator in responseto a control signal; generating a pressure wave in the water in responseto the operation of said motor which pressure wave varies in accordancewith variations in said control signal; detecting said pressure wave inthe water adjacent to said seismic wave generator; generating a feedbacksignal in accordance with said detected pressure wave; continuouslycombining said command signal and said feedback signal in an amplifierto produce said control signal which will generate the desired pressurewave; detecting seismic waves produced by said pressure wave anddeflected from the earth strata beneath the surrounding water with saidseismic wave detector; and recording indications of said detectedseismic waves.
 11. The method of claim 10 wherein said seismic wavegenerator is submerged to a depth of about 21 feet below the surface ofthe water.
 12. An underwater seismic wave source comprising: a commandsignal generator adapted to generate a command signal that varies as afunction of time; motor means driven by said command signal; a housinghaving orifice means for passing pressurized fluid into and out of saidhousing; control means adapted to be actuated by said motor means tovary the flow of fluid out of said housing; and feedback means adaptedto detect changes in the flow of fluid out of said housing and togenerate a feedback signal in accordance with said flow, said feedbackmeans being connected to combine said feedback signal with said commandsignal to produce a control signal whereby said flow of fluid out ofsaid housing varies in accordance with said control signal.
 13. Anunderwater seismic wave source as defined in claim 10 wherein saidcontrol means comprises a valve actuated by said motor means and adaptedto restrict the flow of fluid out of said housing.
 14. An underwaterseismic wave source as defined in claim 13 wherein said feedback meansfurther comprises combining means and a transducer, said transducerbeing positioned outside of said housing at a distance less than about 8feet from said orifice and being adapted to respond to changes in saidflow of fluid out of said housing, said transducer being connected tosupply said feedback signal to said combining means in order to combinesaid command signal and said feedback signal to produce a control signalto drive said motor means.
 15. An underwater seismic wave source asdefined in claim 13 wherein said feedback means is connected to supplysaid feedback signal to modify the respoNse of said control means tosaid motor means.
 16. In an underwater seismic wave source having acommand signal generator, motor means adapted to be driven by commandsignals, a housing having an inlet for passing pressurized fluid intosaid housing and a plurality of outlets for passing pressurized fluidout of said housing, and a valve adapted to drive by said motor means torestrict the flow of fluid out of said housing, the combinationtherewith of: a pressure transducer adapted to detect changes inpressure adjacent to said housing outlets and to generate a feedbacksignal in accordance with said pressure changes, said pressuretransducer being connected to supply said feedback signal to controlsaid flow of fluid out of said housing.
 17. An underwater seismic wavesource comprising: a command signal generator adapted to continuouslygenerate command signals that vary as a function of time; a housinghaving a fluid inlet and a plurality of tubular fluid outlets, saidoutlets being positioned in close proximity to each other; valve meanspositioned in said housing adapted to control the rate of flow from saidinlet to said outlets; motor means connected to said command signalgenerator adapted to be driven by said command signals to move saidvalve means into and out of restrictive engagement with said housing inorder to vary the flow of fluid through said housing; and a pressuretransducer adapted to detect changes in the flow of fluid out of saidhousing and to generate feedback signals in accordance therewith, saidpressure transducer being connected to combine said feedback signalswith said command signals to produce control signals, said flow of fluidout of said housing varying in accordance with said control signals. 18.An underwater seismic wave source as defined in claim 17 wherein saidmotor comprises a servo motor and wherein said pressure transducer ismounted outside of said housing adjacent to said fluid outlets; saidpressure transducer being connected to combine said feedback signals andsaid command signals to produce control signals for actuating said servomotor.
 19. An underwater seismic wave source as defined in claim 17wherein said pressure transducer is mounted inside of said housingadjacent to said fluid outlets in order to detect the pressure inside ofsaid housing adjacent to said fluid outlets.
 20. An underwater seismicwave source as defined in claim 17 wherein said motor comprises a torquemotor and said feedback means comprises a pressure transducer mountedinside of said housing adjacent to said fluid outlets; said pressuretransducer being connected to combine said feedback signals and saidcommand signals to produce control signals for actuating said torquemotor.
 21. An underwater seismic wave source as defined in claim 17further comprising combining means and wherein said pressure transduceris adapted to detect changes in pressure outside of said housing,adjacent to said outlets and to generate feedback signals in accordancewith said pressure, said pressure transducer being connected to supplyfeedback signals to said combining means in order that said commandsignals and said feedback signals may be combined to produce controlsignals to further drive said motor means.
 22. An underwater seismicwave source comprising: a command signal generator adapted tocontinuously generate command signals that vary in frequency as afunction of time; a housing having a fluid inlet and a fluid outlet;valve means positioned in said housing adapted to provide a seal betweensaid inlet and said outlet, said housing providing a valve seat forengagement by said valve; said valve means having an intermediateradially extending flange adapted to engage said valve seat, said valvemeans being connected to said housing by upper and lower diaphragmsealing means; means resiliently urging said valve means intorestrictive engagement with said valve seat; said housing having achamber therein having two closely spaced orifices, one orificecommunicating with the outside of the housing and the othercommunicating with said fluid inlet; flapper means disposed between saidorifices and being adapted to restrict the flow of fluid through one ofsaid orifices at a time; motor means connected to said command signalgenerator and adapted to move said flapper means in accordance withcommand signals received from said generator, thereby controllingpressure in said chamber, said chamber communicating with the outer sideof one of said diaphragms to control the position of said valve meansconnected to said one diaphragm by allowing pressure to be exerted uponsaid one diaphragm to move said valve against said resilient means andout of engagement with said valve seat; and feedback means adapted todetect changes in pressure adjacent to said housing outlet and togenerate feedback signals in accordance therewith, said pressuretransducer being connected to modify the movement of said flapper means.23. An underwater seismic wave source comprising: a rotating drum; amagnetic tape secured to said drum, said tape having zones magnetizedwith a polarity opposite from the polarity of other zones of the tape; amagnetic tape reading head positioned adjacent to said rotating drum,said reading head being adapted to generate electric command signals inresponse to the passage of zones of varying magnetization near saidreading head; a housing having a fluid inlet and a plurality of tubularfluid outlets; a motor mounted in said housing and connected to receiveand be driven by said electric command signals; a valve positioned insaid housing adapted to provide a fluid restriction between said fluidinlet and said fluid outlets, said housing providing a valve seat forengagement by said valve; a surge chamber adapted to receive highlypressurized water pumped from the surrounding water through one end ofsaid surge chamber; said motor being adapted to move said valve into andout of engagement with said valve seat to release controlled amounts ofsaid pressurized water into the surrounding water; a plenum chamberpositioned between said valve and said outlets; and feedback meanssecured to said housing and being adapted to detect changes in pressureadjacent to said housing outlets and to generate feedback signals inaccordance therewith, said feedback means being connected to modify theflow of pressurized water from said housing in accordance with saidfeedback signals.
 24. The method of claim 7 wherein said changes in theflow of fluid are detected by detecting changes in the pressure of thefluid.